1. Field of the Invention
The present invention relates generally to an external interface circuit. More particularly, the invention relates to an interface circuit in accordance with IEEE-1394.
2. Description of the Related Art
Conventionally, in an interface circuit of the kind set forth above, there has been considered a system, in which a plurality of electronic devices are connected through communication control bus in accordance with IEEE-1394 standard and information signals, control signals and so forth are exchanged between the electronic devices through such communication control bus.
Transmission of signals in respective electronic devices within the system has been performed a time division multiplexing per a predetermined communication cycle. Transmission of this signal is initiated in response to transmission of a cycle start packet indicative of start timing of the communication cycle by an electronic device called as a cycle master. It should be noted that the cycle master is determined automatically according to a procedure defined in IEEE-1394 upon connection of respective electronic devices through communication control bus.
As modes of communication within one communication cycle in the system set forth above, there are two kinds of communication modes, i.e., an Isochronous (Iso) communication for an Iso transmission of information signal, such as video signal, audio signal and so forth, and Asynchronous (Async) communication for an Async transmission.
In this case, Iso packet is transmitted in advance of Async packet. The Iso packet can be distinguished from a plurality of Iso data by adding respective channel number. A period after finishing of transmission of the Iso packet to a next cycle start packet is used for transmission of the Async packets.
On the other hand, in the system set forth above, if respective electronic devices are connected by the communication control bus, node IDs (physical addresses) are assigned automatically depending upon a connection mode. The node ID is also re-assigned automatically depending upon new connection mode when new electronic device is added to the system or the electronic device is removed from the system. The communication control bus in accordance with IEEE-1394 has been disclosed in Japanese Unexamined Patent Publication No. Heisei 9-282263.
In the electronic device having the interface circuit in accordance with IEEE-1394 set forth above, a control portion for performing data communication between other electronic devices connected by a daisy chain through the interface circuit, through own device, is driven by a power source of a main body.
A specification of the interface in accordance with IEEE-1394 has been standardized as IEEE-1394-1995. Therefore, detailed description of the specification is neglected from the disclosure.
In the electronic device having the interface in accordance with IEEE-1394, since the control portion for performing data communication between other electronic devices through own device, is driven by the power source of the main body, the power source of the control portion is turned OFF when the power source of the main body is turned OFF.
Therefore, if data communication between other electronic devices connected to the own device through the daisy chain through the own device, the communication is shut off.
The present invention has been worked out for solving the problem in the prior art set forth above. It is therefore an object of the present invention to provide an external interface circuit which permits data communication even when a power source of a main body is cut off while data communication between other electronic devices is performed through own device.
According to the first aspect of the present invention, an external interface circuit comprises:
a control portion performing data communication between other electronic devices connected with an own device through a communication control bus; and
power source control means for switching a power supply from the own device to a power source of the other electronic device when the power supply of the own device to the control portion is cut off.
According to the second aspect of the present invention, an external interface circuit provided in an own device and establishing a daisy chain connection between the own device and other electronic devices through a communication control bus, comprises:
control portion incorporated in the external interface circuit for performing data communication between the other electronic devices through the own device; and
power source control means for switching a power supply from the own device to a power source of the other electronic device when the power supply of the own device to the control portion is cut off.
In the construction set forth above, the power source control means may comprise detecting means for detecting cutting off of power supply to the control portion from the own device, switching means for switching between a power supply to the control portion from the own device and a power supply to the control portion from the other electronic devices, and means for controlling switching operation of power supply by the switching means depending upon a result of detection by the detecting means.
The control portion may be constructed for modulating data to be transmitted between the other electronic devices into data according to a preliminarily set interface standard. The communication control bus may be constructed in accordance with IEEE-1394.
The external interface circuit may further comprise an interface control portion controlling the control portion, and the power source is also supplied to the interface control portion.
If the power source supplied to the interface control portion is cut off, at least data communication between the own device and other electronic device may be cut off on the other hand, if power source is supplied to the control portion, at least data communication between a first other electronic device and a second other electronic device can be continued through the control portion.